Audio systems which receive, process, and transmit digital information are well known in the art. A radio communications system, i.e., one type of audio system, is typically used to exchange information between two remote devices, e.g., a base station and a radio. The transmission medium can be any one of a number of well known transmission schemes, for both wireless and wireline implementation. Typical schemes include conventional frequency modulation (FM), time division multiple access (TDMA), and a wide range of encrypted signal modulation schemes used in the secure communications market.
FIG. 1 shows an exemplary prior art audio system 100. Audio system 100 comprises a controller 102 which controls the timing of data signals being processed by audio circuit 104, via timing signal 106. Audio circuit 104 processes audio signals and communicates with remote audio device 108, via communication resource 110. [It should be noted that audio system 100 might be a sterophonic system wherein remote audio device 108 might be a simple transducer (e.g., speaker), and the communication resource 100 is a simple wireline. Additionally, audio system 100 might be a digital radio communications system which utilizes a wireless communication resource, e.g., radio frequency (RF) channel as discussed above, to exchange RF signals between audio circuit 104 and remote audio device 108 (e.g., subscriber radio)].
Audio systems of this type typically require many different control signals which allow the system components to cooperate and exchange information signals among them. Signalling-intensive environments, however, often produce an undesirable level of noise on the communication resource. This noise may have an adverse effect on the information which is being exchanged over the communication resource. As one example, a peripheral device such as peripheral 112 may require a timing signal from controller 102, which signal may be carried via a printed circuit trace 114. Circuit trace 114, if long enough, behaves much like an antenna--from which electromagnetic energy might radiate (generally depicted by reference number 116). This radiation is a common source of noise interference on communication resource 110.
Another noise contributing source is found in a device that is shared in common by two or more devices. For example, power source 118 shown in FIG. 1 is common to controller 102 and audio circuit 104. In particular, if each of the common devices are drawing current from power source 118 at substantially the same time, the resultant power surge might cause an undesirable noise spike in the response for the communication resource 110.
Yet another source of spurious noise contribution on the communication resource involves the use of digital control, or timing, signals generated by controller 102 to control data signals being exchanged between external data sources and the audio signal processing circuitry within audio circuit 104. An example of such a timing signal is shown in FIG. 2, which generally depicts magnitude (e.g., voltage, current) versus time graph for such a timing signal. Timing signal 200 has a magnitude 201 and is generated by the controller periodically at times 203-206. Period 207 is typically fixed according to a required timing interval for the device being controlled by the signal.
FIG. 3 shows a frequency response characteristic curve for the communication resource. Response curve 300 is shown with noise floor 302 and is meant to be illustrative of a typical frequency response characteristic at any given time on communication resource 110. Frequency 304 corresponds with the frequency of timing signal 200 (i.e., the reciprocal of period 207, expressed in Hz). As illustrated by curve 300, noise level 312 is substantially higher than noise floor 302, and is generally characterized as a noise spur, or spike. Similarly, at frequency 306 (the lowest order harmonic of the frequency of timing signal 200), noise level 313, while less than noise level 312, is still substantially higher than noise floor 302. Likewise, at frequencies 308, 310 (the second and third lowest harmonics of timing signal 200, respectively) exhibit similar spurious noise levels. The resultant peak noise level curve 316 is typically a decaying function of noise level versus frequency. That is, at the higher order harmonics of the timing signal, the spurious noise levels will be indistinguishable from those levels which make up noise floor 302. Accordingly, the information being exchanged anywhere below frequency 310, for example, might be subject to inaccuracies based on the level of noise present on the communication resource at any given time.
There exists a need, therefore, for an audio system that uses timing signals to control data being exchanged within that system, that is not constrained by the shortcomings of the prior art. In particular, a system that did not, through its own control signalling, unnecessarily contribute to the noise level present on the communication resource, would be an improvement over the prior art.